I propose to study the development of vertebrate embryonic sensory nerve cells in vivo. I want to learn when they become electrically excitable, when electrical activity in response to peripheral stimulation first apears, and how their receptive fields develop. The purpose of investigating the sequential appearance of the properties of neurons is to learn how they mature. Previous studies of the development of electrical excitability in embryos have been restricted to muscle cells. The objectives will be achieved by making intra and extracellular microelectrode recordings from the Rohon-Beard cells in the dorsal region of the spinal cord of amphibian embryos. The morphological development of the same cells will be studied in parallel with the physiological investigations, by dye injection and electron microscopy. Cell death is a widespread phenomenon in embryological development; I want to learn what triggers this process in the nervous system. Using the same techniques I plan to study the physiology of Rohon-Beard cells to see what changes in electrical activity precede their degeneration. It has been speculated that the cause of cell death is a failure of nerve cell processes to form or maintain appropriate connections with other cells. The morphology of the dendrites will be studied to see if this is altered prior to or coincident with changes in electrical activity. Particular attention will be paid to the development of dorsal root ganglion cells, which assume the function of Rohon-Beard cells, to learn whether or not they affect the peripheral connections of Rohon-Beard cells. The peripheral connections of the primary sensory cells will also be altered by experimental manipulations of embryos at early stages to see if their degeneration is speeded or slowed.